<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    var str = 'qwebqerqqwe'

    console.log( str[2] )

    console.log( str.charAt(3) )

    console.log( str.charCodeAt(3) )

    // var count = 0
    // var index = str.indexOf('q')  //  0

    // while(index !== -1) {
    //   count++
      
    //   console.log( '出现的索引位置' + index )

    //   index = str.indexOf('q', index + 1)

    // }

    // console.log( 'Q出现的次数' + count )
  </script>
</body>
</html>